The ductus arteriosus (DA) is a fetal artery that allows blood ejected from the right ventricle to bypass the pulmonary circulation in utero. At birth, functional closure of the DA is initiated by a direct, O2-induced vasoconstrictor mechanism. Failure of functional closure can cause persistent DA. This proposal assesses a comprehensive 02 sensing pathway in human and rabbit DA, which is hypothesized to consist of a mitochondrial 02 sensor, which produces a diffusible redox mediator that regulates an effector (voltage-gated K+ channels, Kv). Preliminary data indicate that increasing PO2 hyperpolarizes mitochondrial membrane potential thus increasing H202 production and inhibiting redox-sensitive, Kv channels in DA smooth muscle cells (DASMC). The resulting membrane depolarization activates L-type Ca++ channels, promoting vasoconstriction. Kv1.5 & Kv2.1 are implicated in this mechanism because they are weakly expressed in the premature DA, which constricts weakly to 02, and their overexpression, by gene transfer, enhances DA constriction. Inhibiting certain electron transport chain (ETC) complexes mimics hypoxia: depolarizing mitochondria, decreasing H202 levels, and increasing K+ current (in DASMCs) and relaxing DA rings, consistent with a mitochondrial redox sensor. Sensor and effector pathways are examined in term human and rabbit DA and in models characterized by impaired 02 constriction and decreased Kv expression: preterm rabbit DA and ironically remodeled DA. Laser capture microdissection is used to isolate SMCs from the media of human DAs for analysis of maturational changes in DA genomic and proteomic expression profiles by quantitative RT-PCR and a ProteinChip mass spectroscopy technique. Significance: Early DA closure improves outcomes and promotes early hospital discharge. By understanding the regulation of DASMC Kv channels function/expression, one may be able to modulate the tone/patency of human DA in vivo via drugs or gene therapy. Hypotheses: 1) DA constriction results from inhibition of DASMC Kv channels by a diffusible redox messenger (H202) produced by the DASMC mitochondrial ETC. 2) Maturational changes in the DASMC expression of O2-sensitive Kv channels or -subunits underlies the reduced responses of preterm rabbit DAs to 02 and contributes to their high incidence of patency. 3) Augmenting expression of O2-sensitive, DASMC Kv channels in preterm or ionically remodeled DA (using an SMC-specific adenovirus) will enhance 02 constriction.